The present invention relates to an arrangement in a two-stroke internal combustion engine, especially for a motor saw, the engine having a crankcase, a fan housing including an impeller, and a carburetor provided in a carburetor chamber. The carburetor is connected to the crankcase via an intake passage, and the carburetor chamber is connected via an inlet passage to the fan housing on the outlet side of the impeller.
In two-stroke engines of the aforementioned type, the intake tube from the carburetor is normally connected to the crankcase of the engine via a suitable non-return valve, or via a piston controlled intake port in the cylinder wall. The crankcase and the intake tube define spaces which, from the point of view of aerodynamics, can be referred to as a Helmholz resonator, the resonance frequency of which is related to the geometry of the spaces. Furthermore, some types of motor saws are provided with an arrangement for centrifugal purification of combustion air, wherein the air is conducted via a passage from a zone adjacent to the periphery of the impeller to the carburetor chamber. It is a prerequisite for this arrangement that the carburetor chamber be fairly airtight. The carburetor chamber and the passage define a second Helmholz resonator which is thus connected in series with the first-mentioned Helmholz resonator.
It has appeared that two such oscillating systems connected in series have a negative effect on the filling ratio of the engine at certain speed ranges, resulting in a reduction of the performance of the engine. The factors affected are primarily power and torque, but also the contents of CO of the exhaust gases.